The present invention relates to a knee prosthesis and more particularly to the provision of a knee prosthesis comprising a rotating bearing on the tibial platform with a spine for posterior stabilization of the anterior-posterior translation of the femoral component relative to the tibial component.
The prior art includes various examples of knee prostheses. Examples of posterior stabilized knee prostheses can be found in U.S. Pat. Nos. 4,298,992 and 5,147,405. Also, examples of knee prostheses which provide a rotational bearing can be found in U.S. Pat. Nos. 4,470,158 and 5,395,401. These references are incorporated herein by reference.
The present invention provides a knee prosthesis comprising a femoral component adapted to be implanted on the condylar portions of the femur and having a pair of laterally spaced-apart condylar portions, each of which has an external surface that is preferably smoothly convexly curved in the antero-posterior direction and generally matches the shapes in lateral profile of condylar surfaces of the femur. These condylar surfaces are preferably smoothly convexly curved in all cross-sections along their antero-posterior extent, and the intercondylar portion connecting the condylar portions defines an intercondylar notch having spaced-apart lateral surfaces or walls. This intercondylar notch may preferably be a box-like housing having spaced-apart lateral side walls and an open roof. Within this notch, the femoral component preferably provides an anterior femoral cam and a posterior femoral cam. The prosthesis further comprises a tibial component adapted to be implanted on the tibial platform and including a bearing having on its superior surface a pair of laterally spaced-apart concavities or bearing surfaces, each of which is adapted to receive in nested relation one of the condylar portions of the femoral component. This bearing is preferably formed to include a superior extending spine to be received in the intercondylar notch of the femoral component. The spine preferably has lateral surfaces, an anterior tibial cam, and a posterior tibial cam. A platform is provided to be rigidly attached to the proximal end of the tibia to provide a surface upon which the bearing rotates about an axis generally aligned with the tibia.
In the illustrated embodiment, the relative positions and shapes of the spine and the intercondylar notch of the prosthesis as implanted into the knee joint are such that, when the leg is at or near full extension, where the femur tends to dislocate posteriorly relative to the tibia, the anterior femoral and tibial cams engage each other to prevent posterior dislocation of the femoral component. When the leg is partly flexed, the femoral and tibial cams are spaced-apart from each other and permit relatively free antero-posterior translation of the components but are available to restrain excessive anterior and posterior movements. From approximately 40xc2x0 to 60xc2x0 of flexion to approximately 120xc2x0 of flexion, the posterior femoral and tibial cams engage and bear on each other. Preferably, the posterior cams engage from approximately 50xc2x0 to approximately 120xc2x0 of flexion. During this flexion, the tibial cam prevents the femoral component from moving anteriorly, and the tibio-femoral contact shifts posteriorly. This femoral roll-back theoretically provides for increased range of motion and improved quadriceps efficiency at deeper flexion angles. Further, preferably, the relative positions and shapes of the posterior femoral and tibial cams of the prosthesis as implanted in the knee joint are such that, when the leg approaches full flexion, the tibial cam is of sufficient height to prevent anterior dislocation of the femoral component.
The present invention also provides a method for controlling the antero-posterior translation of a knee prosthesis comprising the steps of providing a femoral component and attaching the femoral component to a femur at its distal end. The femoral component has condyle surfaces which are spaced apart to define a notch therebetween and a femoral anterior cam and a femoral posterior cam disposed in the notch. A tibial component is provided to be attached to the tibia at its proximal end and the tibial component comprises a platform to be attached to the tibia and a bearing mounted on the platform for rotational movement about an axis extending generally in the direction of the tibia. In the method of the present invention, the bearing is formed to provide bearing surfaces for movably supporting the femoral component condyle surfaces. The bearing is also provided with a spine extending superiorly (upwardly) into the notch between the condyle surfaces. This spine is provided for engaging the femoral anterior posterior cams to provide anterior and posterior stability and femoral roll-back. Also, the spine is of sufficient size to provide an adequate subluxation height. The tibial anterior cam preferably inclines upwardly from the anterior portion of the bearing, and the tibial posterior cam preferably inclines downwardly from the peak at a point adjacent the axis of rotation of the bearing and generally parallel to the rotational axis.
The illustrative tibial anterior cam surface inclines upward slightly from the anterior most portion of the bearing to a point at which the inclination is about 40xc2x0 to 50xc2x0 up to the peak of the spine. The illustrative femoral anterior cam is convexly curved to have a curve portion generally aligned with the more inclined portion of the tibial anterior cam from said point to the peak when the knee is at full extension. The femoral posterior cam is also convexly curved, and it is placed to engage the tibial posterior cam when the knee is at about 40xc2x0 to 60xc2x0 of flexion, preferably at about 50xc2x0 of flexion. At that point, the tibial posterior cam prevents the femoral component from further anterior translation. From 40xc2x0 to 60xc2x0 of flexion to 120xc2x0 of flexion, the femoral posterior cam continues to engage the tibial posterior cam, and roll-back occurs during such flexion. Preferably, a spine is provided of sufficient height to reduce the possibility of dislocation from approximately 90xc2x0 to 120xc2x0 of flexion.